Lubricating oils for metal rolling which have been commonly used are oil-in-water (o/w) emulsions prepared by compounding a lubricating component (e.g., fats and oils, mineral oils, and fatty acid esters) and additives such as antioxidants, extreme pressure agents (EP agents), anti-corrosive agents, and oiliness agents, and then emulsifying by the use of emulsifiers. In recent years, with the advance of rolling techniques, rolling speed and also the size of equipment have been increased. The conventional o/w emulsions, however, are not satisfactory for use in these modern techniques, i.e., several problems are encountered in using the o/w emulsions. For example, when it is attempted to increase the amount of oil deposited on a metal to be rolled, the stability of the o/w emulsions is reduced, and, when recycled and reused, they become increasingly unstable and their rolling properties are deteriorated. On the contrary, if the o/w emulsion is excessively stable, sufficiently lubricity is not obtained due to decrease in the amount of oil deposited on the metal to be rolled. When such a stable o/w emulsion is sprayed on the metal to be rolled, a large part of the emulsion is not destroyed and remains in the stable o/w emulsion form and, thus the amount of oil spread on the metal is too low, which results in lubricity failure. That is, a preferred metal rolling dispersion for high speed rolling has stability sufficient to the extent that the dispersion is stable in a feeding line, but when the dispersion is sprayed onto a metal surface, the oil droplets can be easily destroyed, and after the rolling the dispersion condition can be easily revived.
In order to overcome the above problems, British Patent Application Laid-Open Nos. 2,048,934A and 2,115,832A have proposed an improved method. In this method, water-soluble anionic polymers such as polyacrylate salts, and polycarboxylic acid salts (e.g., salts of a methacrylic acid/maleic acid copolymer), water-soluble cationic polymers such as polymers of quaternary ammonium salts of nitrogen-containing monomers (e.g., vinyl pyridine and N,N-dimethylaminopropyl methacrylamide), or water-soluble amphoteric polymers such as copolymers of the above nitrogen-containing monomers and .alpha.,.beta.-unsaturated carboxylic acids or sulfonic acid group-containing vinyl compounds are added to the lubricating oil component (e.g., fats and oils, and wax) and, thereafter, the lubricating oil component is dispersed in the form of particles having a relatively large particle diameter by application of shearing force. The thus-prepared dispersions are then sprayed on the roll and metal to be rolled.
Lubricating oils for metal rolling as prepared by the above improved method, however, are not sufficiently satisfactory. With these lubricating oils, as long as the number of repeated uses is small, the amount of oil deposited is large and they retain their excellent rolling performance (lubricity). If, however, the number of repeated uses is increased, the dispersibility is reduced. This makes difficult uniform deposition of the lubricating oil component and leads to a decrease in the amount of oil deposited; that is, their rolling performance (lubricity) is reduced.